This invention relates to self-adjusting bearings and has particular, though not exclusive, application to such bearings for use in the suspension systems of articulated semi-trailers.
In a plain bearing and shaft assembly there must be physical clearance between the outside diameter of the shaft and the bore of the bearing so that the shaft can be fitted through the bearing.
The relative rotation that then takes place between the shaft and the bearing causes wear of the two components whereby the clearance between the shaft and the bearing can increase to unacceptable limits, at which stage one or other or both of the components must be renewed to restore the clearance to an acceptable value.
One application of such bearings is in the mechanical suspension systems fitted to articulated semi-trailers and the like.
More particularly, such suspension systems commonly comprise two or more axles suspended from the trailer chassis by means of springs and spring hangers. In order to share the load imposed on the suspension amongst the various axles, it is conventional practice to provide an equaliser assembly between adjacent axles, such an assembly including an equaliser which is pivotal about its own central axis relative to an equaliser bracket supporting the equaliser and whereby an imposed load is equally distributed between the associated axles.
The bearing on which the equaliser pivots may be one of two alternative designs.
In a first design, the equaliser is provided with a bore in which a hollow cylindrical bearing is an interference fit, the bearing comprising an outer resilient sleeve to which is bonded an inner sleeve of wear-resistant material. A cylindrical pivot pin extends through the bearing to interconnect the equaliser with its supporting bracket whereby, in use, the equaliser and bearing rotate relative to the pivot pin.
Such an arrangement provides relatively good equalisation of axle loads due to the relatively low friction between the bearing and the pivot pin but, as detailed above, there is considerable wear between the bearing surfaces which necessitates frequent replacement of the bearing to maintain acceptable clearances.
In an alternative design, the bearing comprises a pair of hollow fructo-conical bushes mounted on a cylindrical pivot pin with their tapered surfaces extending longitudinally of the pin and tapering inwardly towards the centre of the pin. The bushes each comprise an outer frusto-conical portion of resilient material and a central sleeve of wear-resistant material bonded into said outer portion, the bushes being housed within correspondingly-tapered bores in the equaliser and the equaliser bracket.
A clamping assembly on the pivot pin compresses the bushes axially whereby the resilient outer portion of each bush is a friction fit in the associated bore, the axially outermost part of said resilient portion with the bore in the equalier bracket and the axially innermost part of said resilient portion with the bore in the equaliser.
In such an arrangement, the rotation of the equaliser relative to its bracket is under the control of the resilient portion of each bush and in particular the torsion set up therein between the axially innermost part of the bush which rotates with the equaliser and the axially outermost part thereof which remains stationary with the bracket.
Although wear between the bearing surfaces does not occur in such an arrangement, a reduced axle-equalisation effect can result due to the torsional rate of the outer portion of the bush. In some instances, the peak loads experienced by the equaliser assembly can overcome the friction between the outer portion of a bush and the tapered bore in which it is received which results in the equaliser rotating relative to the bush and thereafter adopting a rest position angularly displaced from the normal rest position. Thus, once the peak load is removed, the equalisation effect is no longer correct, in that a torsional load imposed on the equaliser by the bush has to be overcome before it can adopt its normal rest position.
In the use of bearings incorporating an outer resilient sleeve to which is bonded an inner sleeve of wear-resistant material, the bearing is commonly an interference fit in an associated bore, and the effect of the bearing on the associated component rotating therein is very dependent upon the accuracy of machining of said bore and the provision therein of a smooth surface for engagement by the outer surface of the resilient sleeve. Accurate machining can be a costly and time-consuming exercise.